This invention relates to time-sampled or time division switching circuits.
Time division switching is based on the principle that a continuously varying information signal may be faithfully reproduced if its amplitude is sampled at least twice as often as the highest frequency variation desired to be conveyed. If a great many information signals are to be sampled, each requiring the same minimum sampling rate for faithful reproduction, the duration of each sample, and hence its energy content, will be quite small. Effective communication thus demands that the energy content of each sample be efficiently utilized.
One technique which ensures that the energy in a speech sample will be efficiently utilized employs resonant transfer circuitry disclosed, for example, in W. D. Lewis U.S. Pat. No. 2,936,337 issued May 10, 1960. Each port circuit appearing in the time division switching system of the Lewis patent is furnished with a low-pass filter having a shunt capacitor, a series inductor for resonating with the shunt capacitor and a sampling switch. The values of the series inductor and shunt capacitance are such that the duration of one-half cycle (or any odd number of half-cycles) at the resonant frequency is an interval of time equal to the interval during which the sampling switch is closed. This permits all of the charge on the shunt capacitor of one port circuit to be transferred through the series inductance and closed switches to the shunt capacitor of the other port circuit.
It is an advantage of the aforementioned resonant transfer system that charge transfer is inherently bidirectional, i.e., the charges present on each of the shunt capacitors just before the switch is closed are interchanged when the switch is closed. This attribute, nevertheless, requires that series inductors be employed in the charge transfer path. Unfortunately, integrated circuit technology does not presently admit of the manufacture of inductances to any practicable degree.
In any physically realizable system having a transmission path of any appreciable length, a certain amount of signal attenuation will be introduced. In D. B. James et al. U.S. Pat. No. 2,936,338 issued May 10, 1960, the attenuation in each direction of transmission is overcome by the use of a respective amplifier. Such an arrangement is often referred to as a "four-wire" transmission path. The gain of the amplifier employed in each path may of course be adjusted to compensate for any loss in transmission between the interconnected ports. However, this leads to manufacturing problems. If each port is to be equipped with a pair of amplifiers, the amplifiers must have their resistance values adjusted in a fairly precise way to achieve the desired amount of gain. Providing for resistance adjustment, however, increases the cost of circuit fabrication.
In addition to conventional time division switching systems, systems are known, as for example from my U.S. Pat. No. 3,860,761 issued Jan. 14, 1975, which employ time sampling techniques in what is, basically, a space division switching system. In the system of that patent, there are multiplexed on a single digital transmission path defined for a particular call, coded representations of the call signaling and network control information and the speech signals, first in one direction and then in the other.
As the desirability of employing time division switching or time-sampling techniques has increasingly suggested itself to designers of switching equipment, the physical size of the average installation and number of telephones covered has tended to increase, thereby tending to introduce the problem of signal transmission delays. It would be desirable to provide a time division switching technique in which the effects of transmission delays could be easily overcome.